An electric rotating machine such as a magneto generator and a brushless motor is constituted by: a magnetic rotor which has a rotor yoke having a magnet attachment surface, and a plurality of permanent magnets attached on the magnet attachment surface of the rotor yoke for constituting a magnetic field; and a stator which has an armature core having magnetic pole portions opposed to magnetic poles of the magnet rotor, and an armature coil or coils which are wound around the armature core.
In such an electric rotating machine, since a magnetic flux interlinking with the rotor yoke is changed along with a rotation of the magnet rotor, eddy-currents flow on a surface of the rotor yoke, and the eddy-currents and an electric resistance of the rotor yoke cause a generation of an eddy-current loss. The eddy-current loss will cause not only a reduction of efficiency of the electric rotating machine, but also an increase of temperature of the rotor. Therefore, when the eddy-current loss is large, an output of the electric rotating machine is limited. Thus, it is preferable to reduce the eddy-current loss as much as possible.
In order to reduce the eddy-current loss, a rotor having a rotor yoke formed by a stack of silica steel boards may be used. However, such a rotor is not generally used since it is not easy to form the rotor yoke of the electric rotating machine with the stack of silica-steel boards.
For reducing the eddy-current loss of the electric rotating machine which has a rotor including a magnetic field, it is known that a large number of grooves are formed on a magnetic pole surface of each permanent magnet as shown in Japanese Patent Application Laid-Open Publication No. 2004-260951.
In case that a large number of grooves are formed on a magnetic pole surface of each permanent magnet, the magnet becomes weak, and the magnet may be easily damaged. In an invention disclosed in Japanese Patent Application Laid-Open Publication No. 2004-260951, the strength of the magnet is prevented from decreasing by coating the magnetic pole surface of each magnet having grooves with resin. However, if the magnet is coated with resin, the process for coating the magnet requires preparations of resin for coating and much time and labor for manufacturing the rotor; therefore, increase in manufacturing cost is unavoidable. Also, if the magnetic pole surface of the permanent magnet is coated with resin, there is a great possibility that the performance of the electric rotating machine may be reduced since a gap between each magnetic pole of the magnet rotor and each magnetic pole portion of an armature core becomes larger for the thickness of the resin coating.